An accelerometer is a type of transducer that converts acceleration forces into electronic signals. Accelerometers are used in a wide variety of devices and for a wide variety of applications. For example, accelerometers are often included various automobile systems, such as for air-bag deployment and roll-over detection. Accelerometers are often also included in many computer devices, such as for motion-based sensing (e.g., drop detection) and control (e.g., motion-based control for gaming).
Generally speaking, a MEMS (Micro Electro Mechanical System) accelerometer typically includes, among other things, a proof mass and one or more sensors for sensing movement or changes in position of the proof mass induced by external accelerations. Accelerometers can be configured to sense one, two, three, or even more axes of acceleration. Typically, the proof mass is configured in a predetermined device plane, and the axes of sensitivity are generally referred to with respect to this device plane. For example, accelerations sensed along an axis parallel to the device plane are typically referred to as X or Y axis accelerations, while accelerations sensed along an axis perpendicular to the device plane are typically referred to as Z axis accelerations. A single-axis accelerometer might be configured to detect just X or Y axis accelerations or just Z axis accelerations. A two-axis accelerometer might be configured to detect X and Y axis accelerations or might be configured to detect X and Z axis accelerations. A three-axis accelerometer might be configured to detect X, Y, and Z axis accelerations.
One category of Z-axis accelerometer uses a proof mass that is configured in a “teeter-totter” or “see-saw” configuration, where the proof mass is supported from a substrate such that the proof mass rotates relative to the substrate under Z-axis acceleration. Sense electrodes placed below (e.g., on the underlying substrate) or both above and below the proof mass, which in many types of accelerometers are capacitively coupled with the proof mass, are used to sense such rotation of the proof mass and thereby to sense Z-axis acceleration. Other electrical components, such as feedback electrodes, also may be included below and/or above the proof mass. U.S. Pat. No. 7,610,809 provides an example of a differential teeter-totter type Z-axis accelerometer having electrodes both above and below the proof mass. U.S. Pat. Nos. 6,841,992 and 5,719,336 provide other examples of such teeter-totter type accelerometers. U.S. Pat. No. 8,146,425 describes a MEMS sensor with movable z-axis sensing element. Each of these patents is hereby incorporated by reference in its entirety.
FIG. 1 schematically and conceptually shows a Z-axis teeter-totter type accelerometer of the types discussed above. In this example, a device chip 102 having a Z-axis teeter-totter type accelerometer with a teeter-totter proof mass 106 and electrodes both above (110) and below (108) the teeter-totter proof mass 106 is mechanically and electrically coupled with a circuit chip 104. The teeter-totter proof mass 106 is configured to rotate about an axis 107 such that the ends of the teeter-totter proof mass 106 are movable in the Z-axis direction. The electrodes 108 and 110 form variable capacitors with the teeter-totter proof mass 106 for sensing rotation of the proof mass 106 and/or imparting forces to the proof mass 106 such as for closed-loop operation and/or self-test. While two electrodes are shown both above and below the proof mass 106 in this schematic drawing, it should be noted that additional electrodes (e.g., feedback electrodes) also may be included in the electrode layers above and/or below the proof mass 106. Thus, for example, each electrode layer may include two or more sense electrodes and one or more feedback electrodes. Various electrical and/or mechanical connections 112 are made between the device chip 102 and the circuit chip 104, such as for electrically coupling circuitry 105 in the circuit chip 104 with the top and bottom sets of electrodes 108, 110 (the electrical connections are shown as dashed lines) and the teeter-totter proof mass 106 (electrical connection not shown for convenience). The accelerometer may be operated, for example, substantially as described in U.S. Pat. No. 7,610,809 (McNeil).
From a fabrication standpoint, the three-layer structure of the accelerometer in device chip 102 (i.e., the device layer containing the teeter-totter proof mass, the electrode layers above and below the device layer, and related structures such as support structures for the proof mass and electrical connections to the proof mass and underlying electrodes) is complex. For example, if the electrode layer above the teeter-totter proof mass is fabricated in-situ with the other structures, then the fabrication process might require many additional steps in order to form the electrode layer above the teeter-totter proof mass, e.g., forming removable structures to support the proof mass, depositing and patterning various material layers on and above the proof mass in order to form the overlying electrode layer with its support structures and electrical connections, and later “releasing” the teeter-totter proof mass, e.g., by etching away one or more layers of protective material used to support and isolate the device layer components during fabrication of the overlying electrode layer. McNeil appears to address this issue by, instead, securing plates or caps to the substrate after fabrication of the device layer and the underlying electrode layer.